This invention concerns the purification of exhaust gases, especially the purification of such gases from diesel and other xe2x80x9clean-burnxe2x80x9d engines.
Diesel engines are in widespread use in all types of vehicles, stationary power sources and naval and commercial shipping. They are very fuel-efficient, but because of their combustion characteristics generate particulate matter (soot, often called xe2x80x9cPMxe2x80x9d) on which a variety of organic substances may be absorbed, including unburnt hydrocarbons (HC) and sulphuric acid produced by oxidation of sulphur dioxide derived from sulphur species present in the fuel or in lubricants. Other engines, such as gasoline direct injection (xe2x80x9cGDIxe2x80x9d), can also produce significant quantities of PM, and we consider that the need for removing such PM will soon be expressed in legislation. Nonetheless, the invention may be applied to combustion processes generally, as well as potentially to chemical process stacks/exhausts, and to combustion engines operating at xcex=1 or greater, or lean-burn engines operating at stoichiometric or rich in order to regenerate some exhaust gas aftertreatment device. For simplicity, however, we concentrate on diesel engines hereinafter.
In order to meet various regulations concerning the level of pollutants, it has become commonplace to fit vehicles with an oxidation or three-way catalyst, which only achieves partial removal of PM. The removal of particulates is generally achieved by using some form of filter or trap, which may be cleaned or regenerated intermittently. It has been suggested to include a catalyst in the fuel to the engine, and as well as platinum group metals (xe2x80x9cPGMsxe2x80x9d), iron, copper or cerium compounds have been suggested. A particulate trap may be catalysed to lower the soot combustion temperature, and some form of external heating, for example electric heating of the trap or of air fed thereto, may be used to initiate soot combustion.
A particularly successful soot trap is marketed by Johnson Matthey PLC as the xe2x80x9cCRTxe2x80x9d (xe2x80x9cContinuously Regenerating Technologyxe2x80x9d) and is described in U.S. Pat. No. 4,902,487. This system uses a conversion of NO in the exhaust gas to NO2, which was discovered to be much more effective at typical low diesel exhaust gas temperatures in the combustion of soot than air or any other exhaust gas component. Thus, NO2 is typically effective to combust PM at about 250xc2x0 C., whereas oxygen is effective at about 650xc2x0 C.
It has been suggested to use a plasma generator for exhaust gas purification (see for example GB 2,274,412 and 2 270 013, UK Atomic Energy Authority). Although it was probably not previously recognised in connection with exhaust gas treatment, such a system produces considerable quantities of NO2. Systems such as previously described do not include any filter or trap in combination with a plasma generator, but we believe that this may be a particularly effective system for treating diesel and similar lean-burn exhaust gases.
Accordingly, the present invention provides a system for treating exhaust gases including NO, nitrogen and particulate matter, which system comprising:
(a) a catalyst for generating NO2 from the NO;
(b) a plasma generator for generating at least one of: (1) NO2 from the NO or nitrogen or both; and (2) ozone; and
(c) a filter for trapping a desired proportion of the particulate matter,
wherein trapped particulate matter is combusted by reaction with at least one of NO2 or ozone. In an illustrative embodiment, the exhaust gases are from a diesel engine.
We believe, although we do not wish to be bound by any theory, that in the present invention NO2 may be generated not only by oxidation of NO in the exhaust gases, but also by oxidation of nitrogen to yield NO, which is itself converted to NO2. In the latter case, there is no reliance upon the quantities of NOx leaving the engine. It is also believed that the present invention is especially valuable in that it is not adversely affected by the presence of sulphur in the fuel or in lubricants, which can poison conventional catalysts.
The invention further provides a method of reducing exhaust gas emissions from diesel engines, comprising oxidising NO in the exhaust gas to NO2 over an oxidation catalyst at oxidation catalyst temperatures within an optimum oxidation catalyst temperature range and using a plasma generator to generate NO2 from NO or N2 in the exhaust gas or both at oxidation catalyst temperatures outside of the optimum oxidation catalyst temperature range, trapping particulate matter on a filter and combusting the trapped particulate matter by reaction with NO2.
In a further aspect, the invention provides a method of maintaining increased levels of NO2 in diesel exhaust gas over exhaust gas emitted from the engine independent of exhaust gas temperature, which NO2 is for combusting particulate matter filtered from the exhaust gas, which method comprising oxidising NO in the exhaust gas to NO2 at an oxidation catalyst temperature within an optimum oxidation catalyst temperature range and using a plasma generator to generate NO2 from NO or N2 in the exhaust gas or both at oxidation catalyst temperatures outside of the optimum oxidation catalyst temperature range.
In yet a further aspect, the invention provides a method of increasing levels of NO2 in an exhaust system to combust particulate matter trapped on a filter in the system, which system includes an oxidation catalyst effective to oxidise NO to NO2 over an optimum oxidation catalyst temperature. The method according to this aspect comprises using a plasma generator to generate additional NO2 from one or both of NO and N2 during engine conditions which are pre-determined to generate increased amounts of particulate matter even when the oxidation catalyst temperature is within the optimum oxidation catalyst temperature range.
The oxidation catalyst can be any catalyst suitable for oxidising NO to NO2 in the presence of oxygen. Typical catalysts for this reaction include platinum on alumina or silica-alumina. The loading of the platinum can be between 0.5 to 200 g ftxe2x88x923, such as 50 g ftxe2x88x923. A washcoat of the platinum and the alumina support can be coated on a ceramic, e.g. cordierite, substrate of 100-600 cells per square inch (cpsi), typically 400 cpsi. Alternatively a metal substrate can be used, in which case typical cpsi values can be up to 1000. The oxidation catalyst can be positioned to treat all or a portion of the exhaust gases upstream of the filter, or may be fitted downstream of the filter to treat all or a portion of the filtered exhaust gas, with recirculation of the treated gas to the filter. The oxidation catalyst can be positioned upstream or downstream of the plasma generator.
The plasma generator may be any suitable type producing a non-thermal plasma, and may be enhanced by electromagnetic radiation. Suitable plasma generators include high voltage (e.g. 20 kV or more) alternating current, preferably pulsed, generators, suitably using two dielectric plates positioned in the gas flow, and piezoelectric devices such as piezoceramic transformers. It may be positioned to treat all or a portion of the exhaust gases upstream of the filter, or may be fitted downstream of the filter to treat all or a portion of the filtered exhaust gas, with recirculation of plasma-treated gases to the filter. In one embodiment of the present invention, a predetermined proportion of the exhaust gases is treated by the plasma to cause substantially all of the NO present to be converted into NO2, and the resulting gases blended with untreated exhaust gases, thus resulting in a desired blend of NO and NO2, which according to some studies, may be more effective for the purposes of the present invention than a gas containing substantially only NO2 in admixture with other exhaust gas components.
The filter used may be a woven or knitted wire filter, a gas-permeable metal or ceramic foamed mass or a wall flow filter of generally known type (honeycomb monolith). For certain vehicles, especially light cars or vans, it may be necessary or desirable, to use a filter design which collects only 80% or so by weight of the total soot particulates and preferably incorporates a by-pass and/or pressure relief valve. The filter may be partially or completely catalysed if desired. A catalysed trap may improve the aggregate removal of pollutants.
An embodiment of the present invention incorporates a means for removing NOx downstream of the filter and plasma generator. Such means may be a NOx trap, which technology is available to the skilled person, and generally includes one or more alkali earth metal compounds, especially calcium oxide or barium oxide, or alkali metal, carried on a metal or ceramic honeycomb-type support. The NOx trap is desirably used in combination with a lean-NOx catalyst. Another means for removing NOx is Selective Catalytic Reduction (xe2x80x9cSCRxe2x80x9d), which is well established for stationary power sources and is receiving increasing attention for vehicular applications. Such a modified system can be effective to meet all current and known future emission control regulations for diesel and like engines.
The plasma generator may be controlled and actuated by an engine management unit, or other microprocessor control unit, to operate intermittently according to certain engine operating conditions (speed, load etc.) which have been pre-determined to generate more soot. The identification of such operating conditions can be readily accomplished by one skilled in the art, and such conditions include times when the emission of particulate matter would be undesirably high in the absence of any treatment therefor.
In one embodiment, the plasma generator is controlled, e.g. by a pre-programmed microprocessor unit in an engine management means, to operate when exhaust gas temperatures are too low for the NO oxidation catalyst to work effectively to produce sufficient NO2 to combust soot on the filter. That is, when the kinetics of the NO+xc2xdO2xe2x86x92NO2 reaction is limited by temperature.
In addition, or in the alternative, the plasma generator can be controlled to operate at higher temperatures, when the NO/NO2 shift is constrained thermodynamically. That is, increasing the temperature leads to a decrease in NO conversion because the reverse of the reaction NO+xc2xdO2xe2x86x92NO2 is competing with the forward reaction.
The temperature range defined by these two limits can be viewed as xe2x80x9can optimum oxidation catalyst temperature range.xe2x80x9d The specific value of this range will depend on a number of factors, such as the type and loading of catalyst and the content of the exhaust gas, and can readily be determined empirically. For many systems, the optimum oxidation catalyst temperature range is approximately 200-400xc2x0 C. , preferably 250-350xc2x0 C. in many cases, and this temperature range is often the temperature of the oxidation catalyst during normal operating conditions. Thus, NO2 can be generated by the NO oxidation catalyst during normal operating conditions, e.g. 200-400xc2x0 C., especially 250-350xc2x0 C. (during which time the plasma generator is switched off) and can be generated by the plasma generator during periods of low or high exhaust gas temperatures at the oxidation catalyst, or both. Alternatively, the plasma generator can be operated to generate additional NO2during engine conditions which generate increased amounts of particulate matter regardless of whether the oxidation catalyst temperature is within or outside of its temperature range.
In each of these circumstances, plasma generated NO2 can be used to maintain soot combustion outside of the normal operating window of the NO oxidation catalyst, and increase the temperature range of application for soot regeneration by combustion in NO2.
Alternatively, the plasma generator may operate during all operational conditions of the engine, which system has the benefit of simplicity, but this may be undesirable if the engine is in an operating condition in which significant quantities of NOx are generated, or during regeneration of a NOx trap.
The present invention, at least in its most preferred embodiments, in addition to being particularly effective at controlling emissions, permits the engine designers to design and tune the engine for power and/or fuel efficiency, rather than being forced to make compromises in engine design to minimise the generation of NOx and particulates. This can be a significant advantage for commercial vehicles, but allows flexibility in design for all engines and types of vehicles.
Yet another embodiment of the present invention is to feed a reductant, which term includes hydrocarbon fuel, e.g. diesel fuel, ammonia, ammonia precursors, hydrogen etc. into the exhaust gases either upstream or downstream of the plasma generator.